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Apache HTTP Server Version 2.4

Apache > HTTP Server > Documentation > Version 2.4

Expressions in Apache HTTP Server

Historically, there are several syntax variants for expressions used to express a condition in the different modules of the Apache HTTP Server. There is some ongoing effort to only use a single variant, called ap_expr, for all configuration directives. This document describes the ap_expr expression parser.

The ap_expr expression is intended to replace most other expression variants in HTTPD. For example, the deprecated SSLRequire expressions can be replaced by Require expr.


See also


Grammar in Backus-Naur Form notation

Backus-Naur Form (BNF) is a notation technique for context-free grammars, often used to describe the syntax of languages used in computing. In most cases, expressions are used to express boolean values. For these, the starting point in the BNF is expr. However, a few directives like LogMessage accept expressions that evaluate to a string value. For those, the starting point in the BNF is string. 

expr        ::= "true" | "false"
              | "!" expr
              | expr "&&" expr
              | expr "||" expr
              | "(" expr ")"
              | comp

comp        ::= stringcomp
              | integercomp
              | unaryop word
              | word binaryop word
              | word "in" "{" wordlist "}"
              | word "in" listfunction
              | word "=~" regex
              | word "!~" regex


stringcomp  ::= word "==" word
              | word "!=" word
              | word "<"  word
              | word "<=" word
              | word ">"  word
              | word ">=" word

integercomp ::= word "-eq" word | word "eq" word
              | word "-ne" word | word "ne" word
              | word "-lt" word | word "lt" word
              | word "-le" word | word "le" word
              | word "-gt" word | word "gt" word
              | word "-ge" word | word "ge" word

wordlist    ::= word
              | wordlist "," word

word        ::= word "." word
              | digit
              | "'" string "'"
              | """ string """
              | variable
              | rebackref
              | function

string      ::= stringpart
              | string stringpart

stringpart  ::= cstring
              | variable
              | rebackref

cstring     ::= ...
digit       ::= [0-9]+

variable    ::= "%{" varname "}"
              | "%{" funcname ":" funcargs "}"

rebackref   ::= "$" [0-9]

function     ::= funcname "(" word ")"

listfunction ::= listfuncname "(" word ")"


Variables

The expression parser provides a number of variables of the form %{HTTP_HOST}. Note that the value of a variable may depend on the phase of the request processing in which it is evaluated. For example, an expression used in an <If > directive is evaluated before authentication is done. Therefore, %{REMOTE_USER} will not be set in this case.

The following variables provide the values of the named HTTP request headers. The values of other headers can be obtained with the req function. Using these variables may cause the header name to be added to the Vary header of the HTTP response, except where otherwise noted for the directive accepting the expression. The req_novary function may be used to circumvent this behavior.

Name
HTTP_ACCEPT
HTTP_COOKIE
HTTP_FORWARDED
HTTP_HOST
HTTP_PROXY_CONNECTION
HTTP_REFERER
HTTP_USER_AGENT

Other request related variables

NameDescription
REQUEST_METHOD The HTTP method of the incoming request (e.g. GET)
REQUEST_SCHEME The scheme part of the request's URI
REQUEST_URI The path part of the request's URI
DOCUMENT_URI Same as REQUEST_URI
REQUEST_FILENAME The full local filesystem path to the file or script matching the request, if this has already been determined by the server at the time REQUEST_FILENAME is referenced. Otherwise, such as when used in virtual host context, the same value as REQUEST_URI
SCRIPT_FILENAME Same as REQUEST_FILENAME
LAST_MODIFIED The date and time of last modification of the file in the format 20101231235959, if this has already been determined by the server at the time LAST_MODIFIED is referenced.
SCRIPT_USER The user name of the owner of the script.
SCRIPT_GROUP The group name of the group of the script.
PATH_INFO The trailing path name information, see AcceptPathInfo
QUERY_STRING The query string of the current request
IS_SUBREQ "true" if the current request is a subrequest, "false" otherwise
THE_REQUEST The complete request line (e.g., "GET /index.html HTTP/1.1")
REMOTE_ADDR The IP address of the remote host
REMOTE_PORT The port of the remote host (2.4.26 and later)
REMOTE_HOST The host name of the remote host
REMOTE_USER The name of the authenticated user, if any (not available during <If >)
REMOTE_IDENT The user name set by mod_ident
SERVER_NAME The ServerName of the current vhost
SERVER_PORT The server port of the current vhost, see ServerName
SERVER_ADMIN The ServerAdmin of the current vhost
SERVER_PROTOCOL The protocol used by the request
DOCUMENT_ROOT The DocumentRoot of the current vhost
AUTH_TYPE The configured AuthType (e.g. "basic")
CONTENT_TYPE The content type of the response (not available during <If >)
HANDLER The name of the handler creating the response
HTTP2 "on" if the request uses http/2, "off" otherwise
HTTPS "on" if the request uses https, "off" otherwise
IPV6 "on" if the connection uses IPv6, "off" otherwise
REQUEST_STATUS The HTTP error status of the request (not available during <If >)
REQUEST_LOG_ID The error log id of the request (see ErrorLogFormat)
CONN_LOG_ID The error log id of the connection (see ErrorLogFormat)
CONN_REMOTE_ADDR The peer IP address of the connection (see the mod_remoteip module)
CONTEXT_PREFIX
CONTEXT_DOCUMENT_ROOT

Misc variables

NameDescription
TIME_YEAR The current year (e.g. 2010)
TIME_MON The current month (01, ..., 12)
TIME_DAY The current day of the month (01, ...)
TIME_HOUR The hour part of the current time (00, ..., 23)
TIME_MIN The minute part of the current time
TIME_SEC The second part of the current time
TIME_WDAY The day of the week (starting with 0 for Sunday)
TIME The date and time in the format 20101231235959
SERVER_SOFTWARE The server version string
API_VERSION The date of the API version (module magic number)

Some modules register additional variables, see e.g. mod_ssl.


Binary operators

With the exception of some built-in comparison operators, binary operators have the form "-[a-zA-Z][a-zA-Z0-9_]+", i.e. a minus and at least two characters. The name is not case sensitive. Modules may register additional binary operators.


Comparison operators

NameAlternative Description
== = String equality
!= String inequality
< String less than
<= String less than or equal
> String greater than
>= String greater than or equal
-eq eq Integer equality
-ne ne Integer inequality
-lt lt Integer less than
-le le Integer less than or equal
-gt gt Integer greater than
-ge ge Integer greater than or equal


Other binary operators

NameDescription
-ipmatch IP address matches address/netmask
-strmatch left string matches pattern given by right string (containing wildcards *, ?, [])
-strcmatch same as -strmatch, but case insensitive
-fnmatch same as -strmatch, but slashes are not matched by wildcards


Unary operators

Unary operators take one argument and have the form "-[a-zA-Z]", i.e. a minus and one character. The name is case sensitive. Modules may register additional unary operators.

NameDescriptionRestricted
-d The argument is treated as a filename. True if the file exists and is a directoryyes
-e The argument is treated as a filename. True if the file (or dir or special) existsyes
-f The argument is treated as a filename. True if the file exists and is regular fileyes
-s The argument is treated as a filename. True if the file exists and is not emptyyes
-L The argument is treated as a filename. True if the file exists and is symlinkyes
-h The argument is treated as a filename. True if the file exists and is symlink (same as -L)yes
-F True if string is a valid file, accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to do the check, so use it with care - it can impact the server's performance!
-U True if string is a valid URL, accessible via all the server's currently-configured access controls for that path. This uses an internal subrequest to do the check, so use it with care - it can impact the server's performance!
-A Alias for -U
-n True if string is not empty
-z True if string is empty
-T False if string is empty, "0", "off", "false", or "no" (case insensitive). True otherwise.
-R Same as "%{REMOTE_ADDR} -ipmatch ...", but more efficient

The operators marked as "restricted" are not available in some modules like mod_include.


Functions

Normal string-valued functions take one string as argument and return a string. Functions names are not case sensitive. Modules may register additional functions.

NameDescriptionSpecial notes
req, http Get HTTP request header; header names may be added to the Vary header, see below
req_novary Same as req, but header names will not be added to the Vary header
resp Get HTTP response header
reqenv Lookup request environment variable (as a shortcut, v can also be used to access variables). ordering
osenv Lookup operating system environment variable
note Lookup request noteordering
env Return first match of note, reqenv, osenvordering
tolower Convert string to lower case
toupper Convert string to upper case
escape Escape special characters in %hex encoding
unescape Unescape %hex encoded string, leaving encoded slashes alone; return empty string if %00 is found
base64 Encode the string using base64 encoding
unbase64 Decode base64 encoded string, return truncated string if 0x00 is found
md5 Hash the string using MD5, then encode the hash with hexadecimal encoding
sha1 Hash the string using SHA1, then encode the hash with hexadecimal encoding
file Read contents from a file (including line endings, when present) restricted
filemod Return last modification time of a file (or 0 if file does not exist or is not regular file)restricted
filesize Return size of a file (or 0 if file does not exist or is not regular file)restricted

The functions marked as "restricted" in the final column are not available in some modules like mod_include.

The functions marked as "ordering" in the final column require some consideration for the ordering of different components of the server, especially when the function is used within the <If> directive which is evaluated relatively early.


Environment variable ordering

When environment variables are looked up within an <If> condition, it's important to consider how extremely early in request processing that this resolution occurs. As a guideline, any directive defined outside of virtual host context (directory, location, htaccess) is not likely to have yet had a chance to execute. SetEnvIf in virtual host scope is one directive that runs prior to this resolution

When reqenv is used outside of <If>, the resolution will generally occur later, but the exact timing depends on the directive the expression has been used within.

When the functions req or http are used, the header name will automatically be added to the Vary header of the HTTP response, except where otherwise noted for the directive accepting the expression. The req_novary function can be used to prevent names from being added to the Vary header.

In addition to string-valued functions, there are also list-valued functions which take one string as argument and return a wordlist, i.e. a list of strings. The wordlist can be used with the special -in operator. Functions names are not case sensitive. Modules may register additional functions.

There are no built-in list-valued functions. mod_ssl provides PeerExtList. See the description of SSLRequire for details (but PeerExtList is also usable outside of SSLRequire).


Example expressions

The following examples show how expressions might be used to evaluate requests:

# Compare the host name to example.com and redirect to www.example.com if it matches
<If "%{HTTP_HOST} == 'example.com'">
    Redirect permanent "/" "http://www.example.com/"
</If>

# Force text/plain if requesting a file with the query string contains 'forcetext'
<If "%{QUERY_STRING} =~ /forcetext/">
    ForceType text/plain
</If>

# Only allow access to this content during business hours
<Directory "/foo/bar/business">
    Require expr %{TIME_HOUR} -gt 9 && %{TIME_HOUR} -lt 17
</Directory>

# Check a HTTP header for a list of values
<If "%{HTTP:X-example-header} in { 'foo', 'bar', 'baz' }">
    Header set matched true
</If>

# Check an environment variable for a regular expression, negated.
<If "! reqenv('REDIRECT_FOO') =~ /bar/">
    Header set matched true
</If>

# Check result of URI mapping by running in Directory context with -f
<Directory "/var/www">
    AddEncoding x-gzip gz
<If "-f '%{REQUEST_FILENAME}.unzipme' && ! %{HTTP:Accept-Encoding} =~ /gzip/">
      SetOutputFilter INFLATE
</If>
</Directory>

# Check against the client IP
<If "-R '192.168.1.0/24'">
    Header set matched true
</If>

# Function example in boolean context
<If "md5('foo') == 'acbd18db4cc2f85cedef654fccc4a4d8'">
  Header set checksum-matched true
</If>

# Function example in string context
Header set foo-checksum "expr=%{md5:foo}"

# This delays the evaluation of the condition clause compared to <If>
Header always set CustomHeader my-value "expr=%{REQUEST_URI} =~ m#^/special_path\.php$#"


Other

NameAlternative Description
-in in string contained in wordlist
/regexp/ m#regexp# Regular expression (the second form allows different delimiters than /)
/regexp/i m#regexp#i Case insensitive regular expression
$0 ... $9 Regular expression backreferences


Regular expression backreferences

The strings $0 ... $9 allow to reference the capture groups from a previously executed, successfully matching regular expressions. They can normally only be used in the same expression as the matching regex, but some modules allow special uses.


Comparison with SSLRequire

The ap_expr syntax is mostly a superset of the syntax of the deprecated SSLRequire directive. The differences are described in SSLRequire's documentation.


Version History

The req_novary function is available for versions 2.4.4 and later.